In the past, rubber which contains α,β-ethylenically unsaturated nitrile monomer units and conjugated diene monomer units or olefin monomer units (nitrile copolymer rubber) has been known as rubber which is excellent in oil resistance. Its cross-linked product is mainly being used as materials for fuel hoses, gaskets, packing, oil seals, and other various rubber products which are used around oils in automobile applications.
On the other hand, in recent years, due to rising global activities to protect the environment, efforts are being made to reduce the amount of evaporation of gasoline and other fuel into the atmosphere. In Japan as well, in fuel hoses, seals, packings, and other applications, not only tensile strength and other mechanical characteristics, but also much better gasoline permeation resistance is being sought.
As opposed to this, Patent Document 1 discloses, as a nitrile copolymer rubber composition which gives a cross-linked product which is improved in gasoline permeation resistance, a nitrile copolymer rubber composition which contains a nitrile copolymer rubber which has α,β-ethylenically unsaturated nitrile monomer units 10 to 65 wt %, conjugated diene units 15 to 89.9 wt %, and cationic monomer units and/or monomer units able to form cations 0.1 to 20 wt % and a flat shaped inorganic filler with an aspect ratio of 30 to 2,000.
However, in the art of this Patent Document 1, to make the obtained cross-linked rubber excellent in gasoline permeation resistance and tensile strength, when preparing the nitrile copolymer rubber composition, it is necessary to mix the nitrile copolymer rubber and the flat shaped inorganic filler in the state of an aqueous dispersion. There was the problem that the production process was therefore troublesome. For this reason, from the viewpoint of streamlining the production process, a nitrile copolymer rubber composition which can give cross-linked rubber which is excellent in gasoline permeation resistance and tensile strength even when directly mixing a nitrile copolymer rubber and flat shaped filler to be included in the nitrile copolymer rubber directly in a dry state has been sought.